1. Field of the Invention
The present invention relates to non-reciprocal circuit elements, and particularly relates to non-reciprocal circuit elements such as isolators, circulators, and the like preferably for use in microwave bands.
2. Description of the Related Art
Non-reciprocal circuit elements such as isolators, circulators, and the like have conventionally had characteristics in which signals are transmitted only in a predetermined specific direction, and are not transmitted in the reverse direction. Using such characteristics, an isolator, for example, is used as a sending circuit portion in a mobile communication device such as a cellular phone or the like.
The elements disclosed in Japanese Unexamined Utility Model Registration Application Publication No. 6-013203 and Japanese Examined Utility Model Registration Application Publication No. 2-018561 are known as three-terminal type circulators, which is one type of non-reciprocal circuit element. As shown in FIG. 2, which corresponds to Japanese Unexamined Utility Model Registration Application Publication No. 6-013203, and in FIG. 5, which corresponds to Japanese Examined Utility Model Registration Application Publication No. 2-018561, the isolation characteristics of the element are monophasic. However, there are cases, depending on the circuit in which the circulator, the isolator, or the like is used, in which flat isolation characteristics are desired, and thus there is a problem in that this type of circulators, isolators, or the like cannot be used in such cases.
For example, in a wireless communication apparatus 100 shown in FIG. 11, a circulator S is provided between a noise canceler NC and an antenna ANT. The circulator S cancels out noise in a reception band by setting the phases of a signal inputted from the antenna ANT and a signal outputted from the noise canceler NC to be inverted by 180° relative to each other. The amplitude characteristics of the noise canceler NC are almost flat, and thus a similar degree of flatness is required in the isolation characteristics of the circulator S as well.
Meanwhile, in an RF circuit 150 of a cellular phone shown in FIG. 12, an isolator I is provided between a power amp PA and a duplexer DPX. A signal outputted from the power amp PA is partially inputted to a gain control circuit AGC via a capacitor C, and an output voltage is monitored. In the case where there is a high amount of reflection from the antenna ANT, the isolator I has isolation characteristics that are low and not flat, and the frequency characteristics are high, the magnitude of power outputted from the power amp PA cannot be accurately detected. Furthermore, the gain control circuit AGC will not operate accurately, and the output of the power amp PA cannot be controlled accurately as a result. Accordingly, flat isolation characteristics are desired in this case as well.